fix_enableBMFF_v1

[clanmills]

In a team discussion on Element following RC2, we agreed that in Exiv2 v1.00, we would:

1. Build the function Exiv2::enableBMFF(bool) into the library (even when -DEXIV2_ENABLE_BMFF=False)
2. The bool returned by Exiv2::enableBMFF(bool) would indicate if the library was built with bmff support.
3. The default for -DEXIV2_ENABLE_BMFF=On
4. The documentation would be appropriated updated.

Built with cmake .. -DEXIV2_ENABLE_BMFF=On

517 rmills@rmillsmm-local:~/gnu/github/exiv2/main/build $ nm -g --demangle lib/libexiv2.dylib | grep enableBMFF
0000000000163c60 T Exiv2::enableBMFF(bool)
518 rmills@rmillsmm-local:~/gnu/github/exiv2/main/build $ make version_test | grep bmff

----------------------------------------------------------------------
Ran 0 tests in 0.000s

OK
enable_bmff          1
519 rmills@rmillsmm-local:~/gnu/github/exiv2/main/build $

Built with cmake .. -DEXIV2_ENABLE_BMFF=Off

528 rmills@rmillsmm-local:~/gnu/github/exiv2/main/build $ nm -g --demangle lib/libexiv2.dylib | grep enableBMFF
00000000001632a0 T Exiv2::enableBMFF(bool)
529 rmills@rmillsmm-local:~/gnu/github/exiv2/main/build $ make version_test | grep bmff

----------------------------------------------------------------------
Ran 0 tests in 0.000s

OK
enable_bmff          0
530 rmills@rmillsmm-local:~/gnu/github/exiv2/main/build $ 

[hassec]

LGTM 👍

[1div0]

OK, makes sense.